Preheat oven to 350 degrees F. Lightly grease a baking sheet with cooking spray.

Halve squash lengthwise and trim ends. Place squash, cut-side down, on baking sheet. Brush with olive oil and season with salt and pepper.

Bake for 20 to 25 minutes, or until squash is fork tender. Remove from baking sheet to cool.

When squash halves are cool enough to handle, scoop pulp from center of each half. Mash pulp and set aside. Return squash halves to baking sheet; set aside.

In a saucepan, combine 1/2 cup water and 2 tablespoons butter. Bring to a boil over medium heat. Remove from heat and stir in stuffing mix. Set aside.

In a large skillet, melt remaining 2 tablespoons butter over medium heat. Add zucchini, celery, and onion; cook for approximately 5 minutes, or until tender. Add stuffing mixture and reserved squash pulp; stir well. Remove from heat. Stir in sour cream and 1/2 cup cheese.

Divide stuffing mixture among squash halves.

Bake for 25 minutes, or until warm and squash reaches desired consistency. Sprinkle squash with remaining 1/3 cup cheese. Return to oven for 5 minutes, or until cheese is melted.